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iwant.fyi - shop, price-check and keep looking

by fyi.iwant
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List the constraint vocabulary

demand.list_constraints
Read-only

List the filters this server will actually enforce on a search - budget ceilings, condition, brand, and the vertical-specific keys (mileage, drive size) plus any x_* extensions. Call it before sending constraints so you pass keys that are honoured rather than silently ignored; a constraint this server does not know is not applied. Free, instant, no side effects. Protocol v1.0 §8.2.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault

No arguments

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
constraintsYesEvery constraint key that can be enforced on a search or want.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changed
    • changedOutput schema / (root)
      Previous value: -nullNew value: +{
+  "additionalProperties": true,
+  "properties": {
+    "constraints": {
+      "description": "Every constraint key that can be enforced on a search or want.",
+      "items": {
+        "properties": {
+          "description": {
+            "description": "What the constraint filters on.",
+            "type": "string"
+          },
+          "key": {
+            "description": "Constraint name, e.g. \"price_max\" or \"condition\".",
+            "type": "string"
+          },
+          "operators": {
+            "description": "Comparisons this key accepts, e.g. lte, eq, in.",
+            "items": {
+              "type": "string"
+            },
+            "type": "array"
+          },
+          "value_type": {
+            "description": "Expected value type.",
+            "type": "string"
+          }
+        },
+        "required": [
+          "key"
+        ],
+        "type": "object"
+      },
+      "type": "array"
+    }
+  },
+  "required": [
+    "constraints"
+  ],
+  "type": "object"
+}
  2. First observed

TDQS

A4.9/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already indicate readOnlyHint=true and destructiveHint=false. The description adds valuable behavioral context: the operation is free, instant, and has no side effects, and unknown constraints are not applied. This goes beyond the structured annotations without contradicting them.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Three tight sentences front-load the core purpose and concrete filter examples, then immediately give usage guidance and the no-side-effects reassurance. No wasted words; every sentence contributes.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's simplicity, empty input schema, existing output schema, and strong annotations, the description is complete. The agent knows what the tool does, why to call it, when to call it, and what to expect behaviorally.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The tool has zero parameters, so the description cannot reasonably add parameter-level meaning. The baseline for 0-parameter tools is 4, and the description fully covers what the agent needs to know about invocation.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states a specific verb ('List') and resource ('filters this server will actually enforce on a search'), enumerating concrete filter categories and x_* extensions. This distinguishes it from siblings like demand.search and demand.capabilities by focusing on the constraint vocabulary rather than searching or capability discovery.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives explicit guidance on when to call the tool: before sending `constraints`, so the agent passes keys the server honors rather than silently ignored ones. It also explains the consequence of passing unknown constraints, making the usage intent unambiguous.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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